1. Field of the Invention
This invention relates to an optical information recording-reproducing apparatus for recording and reproducing information by the use of an optical recording medium such as an optical disk or a magneto-optical disk, and in particular to an apparatus continuously for recording highly dense information.
2. Description of the Prior Art
As an optical information recording-reproducing apparatus of this type, there is one as shown in FIG. 1 of the accompanying drawings. As shown, an intermittent laser beam put out from a laser beam source 101 in conformity with information to be recorded during information recording is reflected by a polarizing beam splitter 102, passes through a quarter wave plate 103 and is condensed on a disk-like recording medium (hereinafter referred to as the disk) 105 by an objective lens 104 and thus, information is recorded. At the same time, the reflected light from the disk 105 passes through the objective lens 104 and the quarter wave plate 103 and is transmitted through the polarizing beam splitter 102, whereafter it passes through an astigmatism optical system 106 and is applied to a four-division light detector 107, and a lens driving device 108 is driven in accordance with an auto-focus signal obtained from the detector 107 to thereby effect an auto-focus operation in which the objective lens 104 is displaced in the direction of the optic axis (the direction of the focus).
The above-described optical systems 101-104 and 106-108 are disposed on a base plate called a recording head 109. This recording head 109 is moved radially of the disk 105 by a head driving unit 110 and a head driving motor 111, and the motor 111 is controlled by a motor control circuit 112 so that the recording head 109 is moved at a predetermined speed. The disk 105 is rotated by a spindle motor 116, which in turn is controlled by a spindle motor control circuit 117 so that it is rotated at a predetermined angular speed.
In such a conventional apparatus, however, there is a limit in the mechanical accuracy of the driving portions such as the head driving unit and the head driving motor, and the track interval cannot be made very narrow under the influence of the eccentricity of the disk resulting from the removal thereof, and this has been a hindrance to highly dense recording.
An optical information recording-reproducing apparatus which is capable of accomplishing more highly dense recording is proposed in British Patent No. 1,465,742. This example is shown in FIG. 2 of the accompanying drawings. A disk 201 provided with a guide track is rotated about a shaft 202 by a motor 203 radially movable by a slide 204. The guide track may be one written in the disk in advance by utilization of the variation or the like in reflection factor, and for example, where information is written in the nth track of the disk, information written in the (n-1)th track may be used as the guide track. An electro-optical modulator 206 is provided in the optical path from a laser beam source 205 to the disk 201. This modulator 206 is controlled by a signal supplied to terminals 207 and 207' so that a light spot affects the photosensitive layer of the disk at a predetermined moment in accordance with a signal representative of information for a predetermined period of time.
A modulated beam 220 is reflected toward a diffraction grating 211 such as a phase diffraction grating by a semi-transparent mirror 209. This diffraction grating 211 divides the beam 220 into sub-beams of different diffraction orders, i.e., a 0-order beam a and two 1 st-order beams b and c. A beam 230 is emitted by a light source 210 such as, for example, a laser. This beam 230 passes through the semi-transparent mirror 209 and, like the writing beam 220, it is divided into three beams of different diffraction orders by the diffraction grating 211.
When information is to be written in the disk, the beam 230 is intercepted by closing the aperture of an apertured flat plate 216, for example. In this case, the beam a is a writing beam. When information is to be read out, the aperture of an apertured flat plate 208 is closed and the aperture of the apertured flat plate 216 is opened to make the beam a into a reading beam.
The image 211' of the diffraction grating 211 is formed, for example, on the focal plane of a lens 215 by a lens 212. Two light spots formed by primary beams b and c are projected onto the edge portion of the guide track during writing and projected onto the edge portion of an information track during reading. The central light spot formed by the 0-order beam a is projected onto the center of the guide track or the information track. Particularly, during writing, care must be taken so that the outputs of the primary beams may be low so that writing may not be effected by these beams.
The beam reflected by the disk is reflected toward a photoresponsive detecting system through a rotatable mirror 214 and a semi-transparent mirror 213. This detecting system comprises detectors 217, 218 and 219.
The detector 217 is a high-frequency information detector which supplies a signal proportional to the accumulated information during reading. By comparing the signals supplied by the detectors 218 and 219, there can be obtained the information regarding the position of the light spots and accordingly the positions of the reading and writing beams associated with the information track and the guide track. The signals supplied by the detectors 218 and 219 are electronically processed into control signals, and by these signals, the mirror 214, for example, can be rotated to correct the positions of the reading and writing beams.
However, in the apparatus as shown in FIG. 2, the use of two laser beam sources leads to a complicated structure of the apparatus, which in turn has led to a problem that the so-called optical pick-up including the light sources and the optical system becomes bulky. This problem will become more serious in the case of an apparatus of the type in which the optical pick-up is moved radially of the disk to thereby effect recording and reproduction than in the case of the apparatus of the type as shown in FIG. 2 wherein the disk is moved. There is also a method in which only a single light source 210 is used and an element 221 capable of changing over the intensity of the beam between the writing level and reading level is disposed in the optical path of the beam 230a, but in this case, the relative position of the primary beam 230a and the secondary beams 230b, 230c on the disk does not vary and therefore, it has been impossible to adopt a beam arrangement by which highly accurate tracking control can be effected both during recording and reproduction.